2. ELECTRICITY
Electricity is a source of energy.
There are various uses of electricity such
as it is used in lightning of our homes, in
industries it is used for working of various
machineries, used in running of electronic
trains etc.
3. ELECTRIC CURRENT
If electric charge flows through a conductor we can say that electric current is in
that conductor. Its direction is from positive terminal to negative terminal.
Ex. In a torch cell provide electric current to glow the bulb.
Electric current is expressed by amount of charge flowing through a particular
area in unit time.
If a net charge Q flows across a cross section of conductor in time t then current
is I, through cross section is
I = Q /t
The electric current is expressed in Ampere ,named after Andre Marie
Ampere. Ammeter measures electric current ammeter must have low resistance.
Small current are expressed in milliampere or microampere
4. ELECTRIC CHARGE
There are two types of electric charge that are positive charge(Ex. Glass rod rubbed
with silk) and negative charge(Ex. Ebonite rod rubbed with woolen cloth).
Opposite charges attract each other negative charges repel each other.
The SI unit of electric charge is coulomb denoted by letter C.
One coulomb is the quantity of electric charge which exerts force of 9 X 109 newtons
on an equal charge of distance of 1 m from it.
1 coulomb has 1/1.6 X 10-19 electrons and if charge is 1.6 X 10-19 C then there is only
one electron.
In an electric circuit the direction of electric current is opposite to flow of electrons.
5. ELECTRIC CIRCUIT
A continuous conducting part consisting of wires and
other resistances like electric bulb etc. , switch
between two terminals of cell or battery along which
an electric current flows is called as a circuit.
6. SYMBOLS FOR ELECTRICAL
COMPONENTS
Scientist have devised some symbols for electrical
components which are easy to draw. They are
electrical symbols or circuit symbols.
7. CIRCUIT DIAGRAMS
A diagram which indicates how different
components in a circuit have been connected by
using the symbols for components is called a circuit
diagram.
8. ELECTRIC POTENTIAL
The electric potential at a point is an electric field defined as the work
done in moving a unit positive charge from infinity to that point.
Potential is denoted by V and its unit is volt. Measured by voltmeter.
A potential of 1V at a point means that 1 joule of work is done in moving
1 unit positive charge from infinity to that point or a potential of 1V at a
point means that 1 joule of work is done in moving 1C of electric charge as
unit of charge is coulomb.
9. POTENTIAL DIFFERENCE
The difference in electric potential between two points is known as potential
difference.
The potential difference between two points in an electric circuit is defined as
amount of work done in moving a unit charge from one point to other point.
Potential Difference = Work Done/Quantity of charge moved
SI Unit of potential difference is volt(V) named after Alessandro Volta, Italian
physicist.
V= W/Q (W= Work done , Q= Quantity of charge moved)
1V = 1J/1Q
Potential difference is measured by voltmeter.
10. OHMS’S LAW
Ohm’s law is the relationship between current (I) and potential difference
(v). It was discovered by George Simon Ohm(German physicist).
According to ohms law at constant temperature the current flowing through
a conductor is directly proportional to potential difference across its ends.
I V (At constant temperature) (I = Current V= Potential difference)
I = R X I (Where R is constant or resistance of the conductor)
V / I = R or V = IR or I= V / R
If potential difference is haled then current is also halved and if doubled
current is also doubled.
11. EXPERIMENT TO VERIFY OHM’S LAW
If we show for a given conductor, say a piece of resistance wire
the ratio V/I is constant then Ohm’s law is verified.
Alternatively if we draw the graph between potential difference
and current we can say that Ohm’s law is verified.
12. VERIFYING OHM’S LAW IN LAB
Suppose we have a resistance wire R(conductor) and we want to verify Ohm’s
law for it. Hence we have to show R obeys Ohm’s law.
For this purpose we take a battery(B) switch(S) rheostat(Rh) an ammeter(A)
voltmeter(V) and connecting wires and make circuit from it.
To start experimenting the circuit is completed by pressing switch and current
starts flowing in whole circuit including conductor R.The current is shown in ammeter
The rheostat is initially adjusted that small current passes through the circuit . The
ammeter reading is now noted. The reading gives us current I flowing through
conductor R.The voltmeter reading is also noted which will give potential difference V.
Now as follows increase the current and note V and I in all the cases
Find ration of V and I in all cases and it is found constant it verifies Ohm’s law.
13. VERIFYING OHM’S LAW GRAPHICALLY
If a graph is drawn between the potential difference
reading and the corresponding current reading of a circuit as
made in the experiment as done before.
If a straight line graph is obtained passing through the
both origin then it shows current is directly proportional to
potential difference.
It is said that Ohm’s law is verified.
14. RESISTANCE OF CONDUCTOR
When the electrons move from one point to another of the conductor to the other part they
collide with electrons and with the atoms and ions present in body of conductor. Due to this
collisions there is some obstruction or opposition to the flow of current through the conductor.
The property of a conductor due to which it opposes the flow of current through it is knows
as resistance.
The SI Unit of resistance is ohm which is denoted by omega ( )
It is necessary to increase or decrease current in electric circuit. A component used to
change resistance is knows as rheostat or variable resistance.
Motions of electrons through a conductor is retarded by a resistance.
When resistance is doubled current is halved when it is halved current is doubled hence
resistance is inversely proportional to current.
15. C O N D U C T O R S , I N S U L AT O R S A N D R E S I S T O R S
On the basis of electrical resistance all substances can be divided into three
groups:-
(i)Conductors:- Those substances which have very low electrical resistance are
known as conductors. They allow electricity to pass through them easily. Ex. Copper
(ii)Resistors:- Those substances which have comparatively high electrical
resistance than the conductors.They are used in devices where high resistance is
required. Ex: Nicrome
(iii)Insulators:-Those substances which have very high electrical resistance are
known as insulators. They doesn’t allow electricity easily to pass through them. Ex:-
Rubber
16. COMBINATION OF
RESISTORS
A part from potential difference current in a circuit depends
upon resistance also hence electrical circuits use comination
of two or more resistors to get required current in circuit.
Resistors can be combined in two ways :-
1) In series
2) In parallel
17. RESISTORS IN SERIES
When two or more resistors are connected end to end consecutively are said to be
connected in series.
The combined resistance of resistors in series is equal to sum of individual resistors.
Ex. Number of resistances R1, R2 then combines resistance R= R1+ R2 .
Resistors are connected in series to increase resistance.
When a number of resistance are connected in series the same current flows through
each resistance which is equal to current flowing in series.
In a series potential difference(V)= Sum of potential difference in all resistors(V1 + V2
+ V3 …)
Current in resistors in series:- IR = IR1 + IR2 ………..
18. RESISTORS IN
PARALLEL
Resistors are connected parallel to each other to reduce the resistances.
The reciprocal of combined resistance of a number of resistances connected in parallel is
equal to sum of reciprocal of all individual resistances.
1/R = 1/R1 + 1/R2 +1/R3 ……`
Derivation:- I = I1 + I2 + I3
I = V/R
V/R = V/R1 + V/R2 + V/R 3
Cancelling V
1/R = 1/R1 +1/R2 +1/R3
Parallel circuit is used in domestic wiring.
19. HEATING EFFECT OF
ELECTRIC CURRENT
When an electric current is passed through a high resistance ire like nichrome wire the resistance
wire becomes very hot and produces it. This is called heating effect of electric current.
This heating effect is obtained by transformation of electrical energy into heat energy.
Consider a current I flowing through resistance R let potential difference be V Let t be the time
during which charge Q flows across . The work done is VQ so source supply energy to VQ in time t
hence, P=V X Q/T = VI
H= VIt
Applying Ohm’s law H= I2Rt
This is known as joule’s law of heating
H= Pt(It is used only if power and time is given)
20. ELECTRIC POWER
Electric power is electrical work done per unit time or it is rate of electric energy
consumed in electric circuit.
1 watt = 1 joule/ 1 second { 1kW = 1000 watts}
Power = Work Done/ Time Taken or P= W/t ………(1)
W= V *I*t joules ……………….(2)
Using (2) in (1)
P = V*I*t /t joules per second
P= VI watts
Electric Power= Voltage x Current
P= I2R
21. PRACTICAL APPLICATIONS OF
HEATING EFFECT OF ELECTRIC
CURRENT
The heating effect of electric current is utilized in working
of electric iron, electric kettle, electric oven, room heater,
toaster, water heaters etc.
The heating effect is used in bulbs
It is used in fuse protecting household wiring and electrical
appliances.
